Recently, superconductivity technology has been progressed remarkably and with an expanding application field thereof, development of a small, high performance refrigerator has become indispensable. For such a refrigerator, light weight, small size and high heat efficiency are demanded.
For example in a superconductive MRI apparatus, cryopump and the like, a refrigerator based on such refrigerating cycle as Gifford MacMahon type (GM refrigerator), Starling method has been used. Further, a magnetic floating train absolutely needs a high performance refrigerator. Further, in recent years, a superconductive power storage apparatus (SMES) or a in-magnetic field single crystal pull-up apparatus has been provided with a high performance refrigerator as a main component thereof. Further, to stabilize the temperature of a component material operating under ultra-low temperatures such as a superconductive wire, superconductive device, infrared ray sensor, the cold accumulating material for extremely low temperature cold for a thermal anchor, heat sink and heat shielding has been widely used.
FIG. 9 is a sectional view showing a main structure of a conventional two-staged GM refrigerator. This GM refrigerator 10 has a vacuum container 13 containing a first cylinder 11 having a large diameter and a second cylinder 12 connected coaxially to the first cylinder 11. The first cylinder 11 contains a first cold accumulating unit 14 which is freely reciprocatable and the second cylinder 12 also contains a second cold accumulating unit 15 which is freely reciprocatable. Seal rings 16, 17 are disposed between the first cylinder 11 and first cold accumulating unit 14, and between the second cylinder 12 and second cold accumulating unit 15 respectively.
The first cold accumulating unit 14 accommodates a first cold accumulating material 18 made of Cu mesh or the like. The second cold accumulating unit 15 contains a second cold accumulating material 19 made of a cold accumulating material for extremely low temperature cold. The first cold accumulating unit 14 and second cold accumulating unit 15 have operating medium (refrigerant) paths for He gas or the like which are provided in gaps of the first cold accumulating material 18 and cold accumulating material for extremely low temperature cold 19.
A first expansion chamber 20 is provided between the first cold accumulating unit 14 and second cold accumulating unit 15. A second expansion chamber 21 is provided between the second cold accumulating unit 15 and an end wall of the second cylinder 12. A first cooling stage 22 is provided on a bottom of the first expansion chamber 20 and further a second cooling stage 23 which is colder than the first cooling stage 22 is provided on a bottom of the second expansion chamber 21.
A high pressure operating medium (e.g., He gas) is supplied from a compressor 24 to the aforementioned two-staged GM refrigerator 10. The supplied operating medium passes through the first cold accumulating material 18 accommodated in the first cold accumulating unit 14 and reaches the first expansion chamber 20, and further passes through the second cold accumulating material (second cold accumulating material) 19 accommodated in the second cold accumulating unit 15 and reaches the second expansion chamber 21. At this time, the operating medium supplies heat energy to the respective first cold accumulating materials 18, 19 so that they are cooled. The operating medium passing through the respective first cold accumulating materials 18, 19 is expanded in the respective first expansion chambers 20, 21 so as to produce cool atmosphere thereby cooling the respective cooling stages 22, 23. The expanded operating medium flows in the respective cold accumulating materials 18, 19 in opposite direction. The operating medium receives heat energy from the respective cold accumulating materials 18, 19 and is discharged. As recuperation effect is improved in this process, the heat efficiency of the operating medium cycle is improved so that a further lower temperature is realized.
That is, in the above described GM refrigerator, the operating medium such as compressed He gas flows in a single direction in a cold accumulating unit filled with cold accumulating materials so that the heat energy thereof is supplied to the cold accumulating material. Then, the operating medium expanded here flows in an opposite direction and receives heat energy from the cold accumulating material. As the recuperation effect is improved in this process, the heat efficiency of the operating medium cycle is improved so that a further lower temperature is realized.
As a cold accumulating material for use in the above-described refrigerator, conventionally Cu. Pb and the like have been used. However, these cold accumulating materials have a very small volumetric specific heat in extremely low temperatures below 20 K. Therefore, the aforementioned recuperation effect is not exerted sufficiently so that it is difficult to realize the extremely low temperatures.
For the reason, recently to realize temperatures nearer absolute zero, use of magnetic cold accumulating material made of intermetallic compound formed from a rare earth element and transition metal element such as Er.sub.3 Ni, ErNi, ErNi.sub.2, ErRh, HoCu.sub.2 indicating a large volumetric specific heat in an extremely low temperature range has been considered.
The aforementioned magnetic cold accumulating material is usually processed to a sphere of 0.1-0.5 mm in diameter to carry out effective heat exchange with the operating medium such as He gas and actually used in the form of a magnetic particle. By applying the cold accumulating unit filled with the spherical magnetic particles to the GM refrigerator, a refrigerating operation to produce an arrival lowest temperature of 4 K is realized.
FIG. 10 is a sectional view showing an example of a structure of a low temperature cold reserving unit 30 using the aforementioned GM refrigerator 10, specifically a cold reserving unit for a superconductive magnet constituting a major part of a superconductive MRI unit, magnetic floating train, superconductive power storage unit (SMES), in-magnetic field single crystal pull-up apparatus and the like.
The low temperature cold reserving unit 30 in FIG.10, comprises a superconductive magnet 31 which is an object to be cooled, a GM refrigerator 10 for cooling this superconductive magnet 31 at ultra-low temperatures, and a plurality of heat shielding members 32 disposed so as to surround the superconductive magnet 31, these components being disposed within the a vacuum container 33. The aforementioned plurality of the heat shielding members 32 are supported in the vacuum container 33 through a supporting member 34. Further, there is provided a heat switch 35 for thermally cutting off a cooling means such as the refrigerator 10 from an already cooled object.
As the aforementioned heat shielding member 32, a copper (Cu) plate having a thickness of 1-2 mm is widely used. To suppress invasion of heat from outside so as to increase cooling efficiency of the overall cold reserving system, the heat shielding members 32 are disposed in multiple layers.
However, different from the conventional GM refrigerator in which the refrigerating cycle is as low as several Hz, in such a refrigerator carrying out high-speed cycle operation like starling refrigerator or pulse tube refrigerator in which the refrigerating cycle is several 10 Hz, a pressure loss in the cold accumulating unit filled with the aforementioned spherical magnetic particles increases so that heat exchange between the operating medium and magnetic particles becomes insufficient. Therefore, the conventional refrigerator has a problem in which a sufficient refrigerating capacity cannot be exerted.
On the other hand, as a measure for reducing the pressure loss in the aforementioned cold accumulating unit, there has been used as a trial such a method in which the magnetic cold accumulating materials formed in the form of a punched plate or ribbon-shaped plate having a plurality of through holes are wound in the form of a roll or such a method in which mesh-shaped cold accumulating materials are stacked in multiple layers so as to form a laminated screen.
However, because the aforementioned magnetic cold accumulating material has a very high brittleness particular in intermetallic compounds, drilling or bending is very hard, and therefore it is substantially difficult to reduce the pressure loss in the cold accumulating unit by the shape of the cold accumulating material.
On the other hand, if a refrigerator is stopped or low temperature liquefied gas such as helium (He) is evaporated in the conventional low temperature cold reserving unit using copper-made heat shielding material, the temperature of the heat shielding material rises for a short time because the specific heat of copper under low temperatures is small so that an effect of preventing heat invasion from outside is lost.
Further, recently, there has been considered a system in which the cooling means is separated from an already cooled object and such an object to be cooled as the superconductive magnet is operated in a compact condition. However, because the conventional heat shielding material made of only metallic material such as copper has a small specific heat, its cold reservation effect is small so that an object to be cooled cannot be maintained at low temperatures for a long time.
As a countermeasure for the above mentioned problem, the inventors of this invention have considered application of magnetic cold accumulating material made of intermetallic compound containing rare earth elements and transition metallic elements indicating a large specific heat particularly in extremely low temperature range such as Er.sub.3 Ni, ErNi, HoCu.sub.2 to composition material of the heat shielding material. However, because generally the magnetic cold accumulating material is brittle, it is very difficult to process to a sheet-like shape having such a size which is used as the heat shielding material.
A cylindrical heat shielding material as shown in FIG. 10 is preferable for an object to be cooled such as the superconductive coil, and however, processing of the brittle magnetic cold accumulating material to a cylindrical shape or curved shape is more difficult as compared to processing to a flat shape.
On the other hand, the magnetic cold accumulating material made of rare earth elements such as Nd has an inferior specific heat characteristic than the magnetic cold accumulating material made of the intermetallic compound. Further, such material has a relatively larger specific heat in extremely low temperature as compared to ordinary metals such as Cu and can be processed to a sheet like shape. However, generally, the heat shielding material is often used in a relatively large area shape and used under a condition in which the heat shielding material is subjected to application of a large load. However, because the structural strength of the heat shielding material made of single rare earth element such as Nd is insufficient, it cannot be applied to the heat shielding material without any treatment.
The present invention has been achieved to solve the above described problems and a first object of the invention is to provide a cold accumulating material for extremely low temperature cold capable of exerting a sufficient refrigerating performance with a small pressure loss of refrigerant (operating medium) and easy to process to a shape reducing the pressure loss, and a refrigerator using the same.
A second object of the invention is to provide a heat shielding member which is capable of preventing an invasion of heat effectively, easy to process to any shape and has an excellent structural strength.